1. Field of the Invention
The present invention relates to a socket for a CPU module, and especially to a two-layer ZIF (zero insertion force) PGA socket whereby a related CPU module may be mounted thereto with zero insertion force and the ZIF PGA socket may be operated to electrically connect with the CPU module without moving the CPU module in a lateral direction.
2. The Prior Art
Conventional ZIF PGA sockets normally comprise a cover defining a plurality of upper passageways therein and slidably engaging with a base which defines a corresponding number of lower passageways retaining contacts therein. The upper passageways and the lower passageways are in constant communication with each other. A cam is received in a space defined between the cover and the base and operative to move the cover along the base thereby positioning the socket at either a loosened status ready for insertion of pins of the CPU or a tightened status forcing the pins of the CPU to abut against the corresponding contacts. When the socket is in the loosened status, the pins of the CPU are inserted into the upper passageways and the lower passageways with a substantially zero insertion force, but are not in electrical contact with the contacts retained in the lower passageways. The cam is then operated to drive the cover to move laterally along the base thereby urging the pins of the CPU module to electrically connect with the contacts of the base. The CPU module is moved by the cover of the socket when the socket is changed from the loosened status to the tightened status.
The CPU module is commonly engaged with a heat sink for heat dissipation. However, due to the high density of modularization, the CPU module is heavy and has a large dimension. Thus, the addition of the heat sink causes the assembly of the CPU module and the heat sink to be larger and heavier whereby operation of the cam to laterally move the CPU module is laborious.
To solve the problem, U.S. patent application Ser. No. 09/138,188, which is assigned to the same assignee as the present invention, discloses a three-layer ZIF socket comprising an upper layer defining a plurality of first passageways for receiving CPU pins when the CPU rests thereon, a lower layer defining a plurality of second passageways for receiving soldering tails therein, and a middle layer movably retained between the upper and lower layers and defining a plurality of third passageways for receiving bridging terminals therein. Each soldering tail has a lower portion soldered on a printed circuit board and an upper portion extends into the corresponding third passageway. The upper and middle layers are dimensioned so that the CPU pin is positioned in the first and third passageways. The bridging terminal is movable by the middle layer to be selectively displaced between a first position where the soldering tail and the CPU pin are not connected, and a second position where the soldering tail and the CPU pin are electrically connected by the bridging terminal. In such a three-layer socket, the CPU pins remain stationary yet can still electrically connect with the printed circuit board via movement of the middle layer. However, the middle layer is apt to warp after manufacture due to its large area thereby adversely affecting the movement between the upper and bottom layers. Moreover, such a three-layer structure requires two kinds of terminals (soldering tails and bridging terminals) which increases manufacturing costs. Additionally, the profile of the three-layer socket is higher compared to the two-layer socket and violates the miniaturization trend of the computer industry.
Hence, it is requisite to provide a low profile ZIF socket which does not move the CPU module laterally when the CPU module is changed between the tightened status and the loosened status.